Objectives: To study the different lymphocytes subpopulation in the peripheral blood of persons who were exposed to x-ray treatment to their thymus gland during their infancy or early childhood and patients with cancer before and after they have been treated with radiation. Approach: The lymphocytes from the peripheral bood are separated by density-centrifugation method or pass through a fiberglass column to remove the phagocytic cells. These lymphocytes are washed with HBSS and the number of T lymphocytes determined by their ability to form rosettes with sheep erythrocytes. The B lymphocytes are then determined from their ability to fluoresce with isothiocynate goat anti-human gamma globulin. All experimental and normal control specimens are coded until analyses are completed. Besides the direct B and T lymphocyte determinations, the subpopulation of lymphocytes are also determined by the indirect method by culturing with phytohemagglutinin or Pokeweed Mitogen. The mitotic rates of these cultures are then determined and their chromosomal pattern determined. Both T and B lymphocytes are decreased in adult patients whose thymus glands were irradiated during infancy or early childhood.